A hallmark of the vertebrate immune response is an ability to Create a diverse repertoire of immunoglobulin specificities. In mouse and human, this is accomplished by the somatic rearrangement of a large number of V(D)J gene segments. By combinatorial joining of gene segments in additional to junctional diversity occurring at the joining sites between gene segments, a spectrum of repertoires is generated. More recently, it has been recognized that these mechanisms of generating diversity are not utilized by all vertebrates. For example, chicken Ig light chain diversity is created by a single V(J) rearrangement followed by gene conversion of the rearranged V(J) gene using, as donors, 25 pseudogenes located immediately 5' to the V(J) gene. Rabbits use a similar strategy of gene conversion to create diversity although it appears that this species carry many more functional V genes than chicken. Sheep light chains also have been shown to undergo few V(J) gene rearrangements, but instead of generating diversity through gene conversion, appear to use somatic mutation in order to create a wide spectrum of specificities. Thus a wide variety of mechanisms are available to vertebrates for generation of diversity. This proposal addresses both the mechanism of generation of diversity and the site where B cell diversification occurs in cattle. Preliminary data are presented to demonstrate cattle express an extremely limited number of V genes and that diversification of these genes occur through either gene conversion or somatic mutation. A last goal of the proposed studies, is to address the control of light chain isotype expression. While data from mouse and human indicate that the preponderance of kappa light chain expression may be explained by the rearrangement of kappa genes before lambda, in species where lambda light chain is the predominant isotype, this question remains unresolved. Through the use of cloned kappa and lambda probes from cattle in conjunction with hybridomas expressing either cow isotype, this issue will be addressed.